Distribution of water vapour. in the atmosphere

Transcription

1 in the atmosphere Bernard Legras Laboratoire de Météorologie Dynamique Institut Pierre Simon Laplace ENS /CNRS/UPMC, Paris, France 1 The origin of water 2

2 There is plenty of water in the universe, produced from combined hydrogen and oxigen produced and released by supernovae. 3 Detail of Eagle nebulae NASA Hubble telescope These clouds contain large amount of water. There is water within the proto-solar nebulae when a new star and planetary system are formed. 4 There are geochimical evidences that most of surface water came with bombing of early Earth by meteorites and comets. This also explains the abundancy of noble (siderophil) metals in the crust. 0,3% of Earth mass was added this way. There was water on the Earth during its early stages (formation of zircons By) and oceans were present My after the formation of the Earth. However, most of water was not contained within the accreted cloud. protosolar nebulae crust oceans carbonated chondrites micrometeorites deuterium/hydrogen (ppm) comets

3 Magritte, Zeno 5 Thermodynamical constrains 6

4 7 Earth conditions are such that water is present at the surface and in the atmosphere under its three phases. Moisture condensation Atmospheric water vapour is characterized by its partial pressure e, and its mass mixing ratio r = ρ v /ρ d = (e/p d )(R d /R v ) where d refers to dry air. Sometimes, use volume mixing ratio e/p d Saturated pressure ratio depends exponentially on temperature (Clausius- Clapeyron law). Empirical fits (T in K): e s liquid = 6,112 exp(17,67 (T )/(T-29.65)) e s ice = exp(23, ,72784/t + 0,15215 Ln(T)) Exemples of saturated ratio at 1000hPa and T=20 C: r s = 14,5 g/kg, at 800 hpa (2000m) et T = 7 C: r s =7,8 g/kg, d log e S dt = L R v T 2 Adiabatic ascent of an air parcel from the ground r s (z) Initially p 0, T 0, r 0 r s T 0 p 0 p z R/C p, p z 8 at 500 hpa and T=-30 C: at 100 hpa and T =-80 C: r s =0,47 g/kg, r s =0,003 g/kg, (the atmospheric water vapour content is divided by almoste four orders of magnitudes between sea level and 100 hpa in the Tropics) r s (T 0, p 0 ) LCL (lifting condensation level): condensation leval of parcels lifted from the ground r s

5 Water vapour drops to very low values (3-4 ppmv, 2-3 mg/kg ) at the tropical tropopause due to cold point near 190K. tropopause hygropause Fueglistaler et al., Rev. Geophys., 2009 Sensitivity to temperature is 0.5 ppmv K and atmospheric circulation 11

6 12 ECMWF ERA-Interim Perrehumbert, CUP, Surface upward longwave radiation - outgoing longwave radiation

7 Total water vapour column and greenhouse effect are related to sea surface temperature but not as a simple function. 15 ECMWF ERA-Interim

8 16 E ~ C V (1 H) r S where H is relative humidity r/r S above the surface. E is strongly constrained by the net radiative flux at the surface. E is mostly distributed over the winter subtropics. Transport to and convergence in the ITCZ by trade winds and lower branch of monsoon circulation. 17 Specific humidity

9 18 Relative humidity H=r/r S 19 In the mid and high latitudes, poleward isentropic motion is accompanied by upward motion. In the tropic, vertical upward motion needs heating.

10 Distribution of relative humidity in the toposphere according to analysis of weather centers Detrainment of tropical convection Courtesy of D. Jackson Subsidence branch of the Hadley cell 20 Moistening by sloping motion associated with Rossby waves and baroclinic perturbations Deep convection in the tropics T 200K TTL Folkins and Martins, Injection of water in the upper troposphere is due to convection. Mid -troposphere is moistened by evaporation of precipitations, descent of detrained air and exchanges with extra-tropics.

11 Relative humidity rises to 80% with frequent supersaturation near the tropopause 22 Fueglistaler et al, Rev Geophys, 2009 Thin cirrus (T<-40 C, optical depth < 0.3) Average from CALIPSO June-July-August E. Martins, 2009 Thin cirrus extend over a large range in the tropics under the tropopause where relative humidity is large. 23

12 Development of an idealized baroclinic perturbation = pression = température Cold front Warm front Air motion associated with a frontal system. 24 Visible channel Meteosat 25 Although mixing and convective instability do occur during the development, the main ingredient is adiabatic baroclinic instability, that is isentropic motion. Water vapour channel Meteosat Descending dry air Ascending moist air generating clouds and precipitations.

13 Appenzeller, Davies & Norton, 1996, JGR, D101(1), PV tracer reconstructed by advection at 320 K Meteosat water-vapour Water vapour distribution is strongly constrained by transport Reconstruction of tracer fields Reconstruction of a tracer field is obtained by reverse time integration of particle trajectories initialised from their final position from t 0 to t 0 - τ, using analysed winds (e.g. ECMWF winds). - assignment of the chemical tracer value or PV at t 0 - τ location from low resolution CTM (chemical transport model) or analysed fields Time T Regular grid (domain filing) or aircraft track Time T- T velocity u x a,t,t tracer concentration c x a,t,t 27

14 The last saturation paradigm 28 The last saturation paradigm has been used in a number of studies. The hypothesis is that relative humidity could be entirely explained by the temperature and pressure history of the parcel. Namely, the humidity content is determined by the lowest saturation ratio during the past history of the parcel. This principle has been applied to the entry of water into the stratosphere and to the distribution of water in the troposphre. It works strikingly well in the first approximation. z Last saturation (Tm,pm) H= p r S (T m, p m ) p m r S (T, p) 29 Actual location t

15 Observed brightness at 6.3µm Calculated brightness 30 Pierremhumbert & Roca,

16 32 In the tropics parcels encounter their last saturation just after being detrained from a convective cloud. In the subtropics, they can move up and down along isentropes under the action of Rossby waves and get saturated near the tropopause. Other complications: gravity waves, heating/cooling induced by clouds Preservation of the relative humidity under global warming Most climate models predict weak variations of H under global warming. Why? Assuming that a parcel at T, p has been last saturated at T m, p m Assuming a uniform warming T and that the trajectories are unchanged, then the parcel at T T, p has been last saturated at T m T, p m and its relative humidity is H T = p e S T m T p m e S T T Using the Clausius-Clapeyron law H T =H 0 1 L R v T T 2 T m 2 1 T T With T =260 K,T m =240 K and T=3 K, we get H T H 0 H 0 =0.042 Important consequence: water vapour feedback on climate change is positive. 33 After Pierrehumbert, Roca & Brogniez, 2007

17 Even for fairly large uniform warming, the variation of H is small But Warming is not uniform: due to the shape of the moist adiabatic, warming is larger in the upper troposphere For those parcels which are last saturated when they are detrained, the detrainment temperature my not change as the climate warms (Fixed Anvil Temperature, Hartmann) The Hadley cells gets weaker and wider with global warming, hence circulation is changing. 34 Change of H in the CMIP3 models (IPCC AR4). Sherwood et al., JGR, doi: /2009jd012585,

18 Stratospheric water 36 Stratospheric water vapour 37 Minimum of water vapour just above the tropical tropopause. Source of water vapour in the stratosphere from the oxidation of methane

19 Troposphere Stratosphere The Brewer Dobson meridional circulation Tracer isopleth Polar vortex Iso potential temperature TTL Heating θ=380k Vertical gradient Jet Horizontal gradient Isentropic mixing θ=350k Cooling Tropopause 38 Tropics Mid-latitudes Pole The Brewer-Dobson circulation from the point of view of heating rates 39 Fueglistaler, Legras et al., QJRMS (subjudice), 2008

20 Tape recorder of H2O in the tropical stratosphere Courtesy of Rosenlof and Reid, ERA-Interim versus MLS-HALOE: - tape recorder is too fast in the lower stratosphere (too large diffusion?) - too dry during summer - lack of methane conversion - smaller dry trend 41

21 Water vapour variation in the extratropical stratosphere. 42 Solomon et al., Science, Effect of water vapour decline in the stratosphere after 2000 may explain in part the levelling of warming during the last ten years. Solomon et al., 2010

22 45 The TTL has long turnover time (> 1 month). Hence, local processes are important.

23 The slow ascent hypothesis versus the overshoot hypothesis TTL TTL 46 Although a very small number of clouds reach the tropopause (<0,5%), the impact on water vapour might be noticeable. Overshoots in cloud resolving models: km - duration 10-20' - max speed 80 m/s - inject 6t/s of water up to 410K 47 Chaboureau, 2007

24 48 Courtesy of T. Roeckmann Clouds in the TTL

25 Convective Sources-to-100hPa trajectories (CS-100hPa) 51

26 Backward Lagrangian trajectories in the TTL from 100 hpa during monsoon season. Diabatic heating rates from ERA-Interim Rightness temperature from CLAUS James et al., GRL, Distribution of convective sources 53

27 54 Clustering of intersections of backward trajectories with mesoscale convective systems. Corti et al., ACP, 2005 James et al., GRL, Clouds aid the transition across the clear sky Q=0 boundary. Do they play a role in the subsequent ascent?

28 Distribution of transit times across the TTL Area distribution of sources with brightness temperature lower than a given threshold among the are of convective system reaching this threshold (%) 56 S e Sensitivity studies Reference - Simulation with vertical velocities (instead of heating rates) Reference - Simulation ignoring cloud tops 57 v ppmv Overshoot parameterization: encounter probability with exponential decay with height adjusted to be 6% over ocean and 15% over land 1km above cloud top (10 times Liu & Zipser, 2005). Frequency needs to be multiplied by 100to get 0.3 ppmv) Reference - Simulation with clear sky heating rates (instead of all sky) Simulation including overshoots - Reference

29 Water isotopologues 58 Isotopic ratio is the main paleo-thermometer used to retrieve temperature from ice records. Adapted from Roeckmann,

30 Rayleigh distillation 60 Water isotopologues concentrate within condensates. If condensates are removed by precipitation, the humid air parcel is depleted. If condensates are lifted within the air parcel and subsequently evaporate, there is no depletion. Hence, depletion provides information on processes occuring within clouds and helps to improve parameterizations. 61 ACE: Tropical (15S-15N seasonal average (roughly 200 profiles each season transformed into theta coordinates See papers by Nasser et al. ICOS: ca. 12 tropical profiles out of Costa Rica (CR-AVE mission. ICOS uncertainty up to 80 per mil, 2006 Jan 27-Feb 11, 90 min. ascent pruned Courtesy of L. Moyer & B. Randel

31 Bolot, Legras, Moyer et al., Rayleigh curve: slow distillation, condensates are removed from lofted parcel; Vapour at equilibrium with ice in undiluted ascent where ice is lofted with air and evaporate. 63

32 TC4 campaign, July-August

33 Bolot, Legras, Moyer et al., 2011b 66 Combining isotopic measurements with crystal shapes and sizes allows to attribute depletion to shapes. Large depletion for small crystals suggests that nucleation occurs in the top of convective towers. Using a model, altitude of nucleation can be retrieved. Water vapour database 67

34 68 Hocke, 2009